Fuel injectors are critical components of gas turbine engines. A fuel injector serves to convey liquid fuel from a manifold delivery system outside of the combustion zone, through a region of very hot air, and ultimately into the combustor through a nozzle. A typical fuel injector receives fuel from a manifold through an inlet fitting on one end, carries the fuel through a fuel tube disposed inside a bore of the injector support, and delivers fuel to the combustor of a gas turbine engine through an outlet fitting and nozzle on the other end. Ordinarily, the fuel tube is rigidly connected, or fixed, at both the inlet fitting and the outlet fitting.
Problems arise due to this fixed connection at both ends of the fuel tube. During engine operation, the air outside the fuel injector, to which the injector support is exposed, is in excess of 1000° F. (538° C.). The fuel tube inside of the injector support, however, is insulated by an air gap, as it must be kept below 400° F. (204° C.) to prevent fuel coking. This difference in temperature leads to differential thermal expansion of the injector support and the fuel tube. Because the fuel tube ordinarily is fixed at both ends inside the injector support, when the injector support thermally expands more than the fuel tube due to exposure to higher temperatures, the fuel tube is imparted with high stresses at the fixed connections and can fail. Therefore, the injector support must be allowed to thermally expand without causing a failure in the fuel circuit. This is especially true within modern gas turbine engines, where temperatures continue to increase.
Efforts have been made to solve this problem. Most of these efforts have centered on designing fuel tubes with coiled or helical portions, as shown for example in U.S. Pat. No. 6,276,141 to Pelletier. Another solution to compensate for differential thermal growth of the injector support and the fuel tube during engine operation has been the addition of a structure joined to the inlet end portion of the fuel tube, as shown for example in U.S. Pat. No. 7,900,456 to Mao. Although such elaborate fuel tube geometries and additional components may prevent failure in the fuel circuit due to differential thermal growth during engine operation, significant costs are incurred in making these fuel tubes.